This invention relates to an apparatus and method for post weld heat treatment of large tubular heat exchangers, and more particularly for post weld heat treatment of nuclear steam generator components.
High pressure heat exchangers are usually manufactured by welding together a plurality of metallic members, including relatively thick cylindrical shell sections, headers, tube sheets and a bundle of tubes. Governing Codes often require welded sections to be heat treated in order to relieve stresses. In addition to meeting the governing codes, it may be required to heat treat the complete tube bundle to reduce residual stresses in the tubes due to manufacturing operations. This reduction of residual stresses may have metallurgical benefits to the life and integrity of the tubing in service. One of the most common mehtods of stress relieving large heat exchangers has been to heat the exchanger inside a furnace. Pursuant to this method hot gases contact the outer surfaces of the exchanger, and the exchanger thereby absorbs heat. This method not only requires that the exchanger be heat treated for a relatively long time, in some cases a week or more, but also may create large temperature differentials between some of the parts of the heat exchanger. For example, large temperature differentials may exist within the tube bundle between the inner and outer tube regions and also in the tubesheet. These large temperature differentials can be experienced when the heat exchanger is being heated up to a desired temperature level, while it is being held at a desired temperature level, or while it is being cooled down to ambient temperature. As a consequence of these temperature differences thermal stresses can be induced in parts of the heat exchanger. Although longer and slower heat treatment in a furnace may lessen the likelihood of inducing thermal stress, it would increase the time during which the shell portion of the heat exchanger is exposed to heat, and consequently the strength and impact properties of the shell would deteriorate. An additional consequence of the temperature differentials could be that the required heat treatment temperature is not reached in all regions of the tube bundle.
The present invention provides an apparatus and method for heat treating heat exchangers, including nuclear steam generators and components thereof, whereby large temperature differentials between heat exchanger elements can be eliminated, yet heat treatment can be completed in a relatively short time.